Uninstall old whonix-14 Reinstall new whonix-15 search this forum
then ask
https://www.whonix.org/wiki/Qubes/Install
it may look more complicated than it is ; because of the formatting
start at the start , learn the terminology , your just changing the
appvms to not reference the whonix-14 templates then removing the
templates via dom0
and either running the script or reinstalling the same way most
templates are installed via dom0
only trick may be changing the "jinja" config file
from -14 to -15
though if you read the thread some folks didn't remove -14 before
installing -15 and apparently that may be "safe" also